1. Field of the Invention
The present invention relates generally to isolators, and in particular, to high voltage ion propellant isolators.
2. Related Art
High power ion propulsion systems or thrusters produce thrust by accelerating a beam of positive ions through an electrostatic field to high velocities, positive ions are produced by electron bombardment of neutral propellant atoms in a discharge chamber. The discharge chamber is typically a cylindrical anode, with a centrally located axial hollow cathode. Typically, the cathode is heated to enable thermionic emission of electrons. Once cathode emission is established, a low current, low voltage discharge between the cathode and anode accelerates electrons into the discharge chamber. A magnetic field is applied to the discharge chamber which increases the electron path length and residence time in the chamber, and thus collision probability.
Propellant atoms (typically noble gases, such as Xenon) are injected into the chamber and collide with energetic electrons. These collisions remove additional electrons from the atoms, resulting in positive ions. A series of two or three perforated electrodes (called grids) attract the positive ions, accelerate them, and focus them into an ion beam. Finally, a neutralizer emits exactly the same number of electrons into the beam as there are ions, which prevents a large negative potential from building up.
Ion engine propellants are chosen for a combination of low ionization potential, high atomic weight, handling, and storage properties. However, lighter noble gases, such as krypton and argon, result in poor discharge chamber performance, increased erosion rates, and increased power required for a given thrust. Using xenon, on the other hand, allows major simplifications in the design of the thruster, its power processing unit, and its propellant feed system, but at a higher cost. Thus, krypton and argon are lower-cost alternatives, but result in lower performance and engine life relative to xenon.
Currently, xenon ion thrusters are desirable for use in spacecraft, such as satellites. One reason is the electrostatic acceleration process in ion propulsion is almost 100% efficient. In practice, the acceleration efficiency is typically 99.7%. This nearly lossless acceleration mechanism enables the development of ion engines which can process megawatts of input power while maintaining reasonable engine component temperatures without active cooling. Xenon ion thrusters are also capable of processing input powers from tens of kilowatts on up at impulses of thousands of seconds.
As the gas moves between ground and a high potential, ionization occurs, which can lead to uncontrolled current conduction through the gas, current flow is minimized by providing an electrical isolator between the two widely different potentials, such as between the gas source and the ion source. The current generation of isolator used for xenon delivery systems is based on utilizing segmented isolation, in which each segment consists of a metal screen separated with a ceramic washer. Isolators for current xenon thrusters utilize a stack of these segments, e.g., 8 to 13, to achieve the necessary voltage standoff. FIG. 1 shows a typical propellant isolator 100 for use in ion thrusters or propulsion systems. Isolator 100 includes an outer shield 102, an inner shield 104, an isolator housing assembly 106, and a stack or series of 13 ceramic isolator rings 108, each followed by a steel mesh screen 110. An arrow 112 shows the direction of gas flow through isolator 100. Current isolator designs, such as shown in FIG. 1, allow xenon to flow in a straight path through the segments. As a result, the xenon “sees” a path length that is roughly the length of the isolator.
However, as xenon ion thruster technology moves toward higher powers and accelerating voltages, the need for greater electrical isolation between system components increases. Next generation thrusters will require much higher voltage standoff, which may necessitate three to five times the number of segments of current designs. Consequently, isolators of current designs meeting the higher voltage standoff requirements would be larger, heavier, and more complex to assemble than present day isolators, such as shown in FIG. 1.
Accordingly, there is a need for a propellant isolator that is capable of higher electrical isolation without greatly increasing the size of the isolator.